You really believe a phone is truly switched off? Unless the main processor kills the baseband power by GPIO, it is not - and baseband processors are a fairly easy exploit target, much more so for a state-level threat.
I can leave it at home or buy a phone that has hardware validation processes to ascertain its correctness when turned off (or the transmitter system is disabled). I could put the phone in a Farraday cage or selectively raise the noise level of certain broadcast ranges within a limited range. I could kill the radio-IC and use a wifi mesh network. All of these measures are not prevented by federal law.
Federally mandating trackers in a car is a completely different story. It will be illegal to remove these trackers, so a basic liberty will be taken away.